Radar systems frequently display received data in the PPI (Plan Position Indicator) mode of display operation. In such systems, data in the form of radar target echoes is written onto the cathode-ray tube display screen as the cathode-ray tube beam is swept outward from its start position by intensifying the cathode-ray tube beam during the time radar echoes are being received. The sweeps are started from the center at the same angle or azimuth as the radar antenna. Early methods for producing this type of radar display included one wherein a deflection coil mounted on the neck of the display cathode-ray tube was rotated at the same rate of rotation of the radar antenna. Synchronization between the deflection coil and radar antenna was maintained by driving both the radar antenna and deflection coil from the same motor shaft or by providing a separate motor for the deflection coil which in turn was connected in a servomechanism loop with the radar antenna. Current passed through the deflection coil caused the beam to move outward from the center of the screen.
Problems in those types of systems were numerous and difficult to overcome. Mechanical problems resulted when the deflection coil was mechanically linked to the radar antenna driving mechanism. Much maintenance was needed and typically a great deal of distracting noise was generated in the display console. Servomechanism driving loops tended to be expensive, take up large amounts of space, have a great deal of weight, and tended to be difficult to maintain. Moreover, in any of the rotating deflection coil systems it was not possible to start the deflection of the cathode-ray tube beam at other than the center of the cathode-ray tube. This made the system difficult to use in many applications, especially when it was desired to view only a portion of the radar swept area as a magnified view.
Difficulties of rotating deflection coil systems were partially overcome by providing a cathode-ray tube display system with two stationary deflection coils with separate drive and amplification circuitry for each coil. An input signal applied to one channel of such a display system caused the beam to be deflected in the horizontal or X direction from the center of the screen while an input signal applied to the other channel caused the beam to be deflected in the vertical or Y direction. Means was provided at the radar antenna for sensing the pointing angle relative to north or azimuth of the radar antenna and converting the azimuth to X and Y signals to deflect the beam of the cathode-ray tube.
In one such system, a rotatable variable capacitor was connected to the rotating radar antenna shaft wherein the capacitance was varied as a function of the antenna azimuth. Circuitry was provided to convert the capacitance value to X and Y signals representing respectively the cosine and sine of the radar antenna azimuth. A ramp waveform was generated and the amplitude of the ramp multiplied or modulated by the cosine of the azimuth for the X channel and by the sine of the azimuth for the Y channel. The ramp signal was started by the same triggering signal which caused the radar transmitter to generate the transmitted radar pulse. The ramp signals as so modulated were then amplified and coupled to the X and Y deflection coils. This system overcame some of the mechanical problems of the previously described rotating deflection coil systems but further problems were introduced. The magnitude of the capacitance used and the magnitude of the capacitance changes effected by the rotation of the radar antenna were relatively small. As the radar antenna is located in a mechanically and electrically noisy environment, noise was frequently introduced into the system resulting in positional inaccuracies caused by distortions of the cosine and sine values of the azimuth.
More modern systems have employed digital azimuth converters. In some of such systems, the 360.degree. radar antenna rotation is divided into a number of small sectors and a pulse is generated as the antenna passes the boundary between sectors. These pulses are termed in the art ACP's (azimuth change pulses). Additionally, a second pulse is generated each time the radar antenna passes a chosen reference such as north heading. This pulse has been termed the ARP (azimuth reference pulse). A digital circuit is then provided which counts the number of ACP pulses which have occurred since the preceding ARP pulse and thereby generates the sine and cosine values knowing at which angles the ACP pulses occur. The earliest and most straight forward method for digitally producing these sine and cosine values was to store sine and cosine values directly for each of the possible sector boundary crossings. Large amounts of storage were thereby required as typically several thousand sector crossings were typically employed and typically 15 or more bits of accuracy for sine and cosine values were required to achieve acceptable presentations on typical sizes of radar display cathode-ray tubes. Another method employed was to store the values of the differences of adjacent tabular values of the sine and cosine functions and to add these differences to the preceding values of sine and cosine respectively to obtain a present value of these functions. Although this method decreased the required storage capacity, still relatively large amounts of storage were required to obtain the 16 to 18 bit accuracies required for large screen radar displays, especially those displays in which it is desired to offset the sweep start from the center of the cathode-ray tube screen and display only a portion of the radar swept area.